For traditional isolated switching converters as shown in FIG. 1, an optocoupler OP_CO and an adjustable shunt regulator TL431 are commonly utilized to feedback the output status to the primary side, so as to regulate the output voltage Vout. The light emitting device in the optocoupler OP_CO has one terminal coupled to the output voltage Vout through the resistor R2, and another terminal coupled to the secondary reference ground through the shunt regulator TL431. The photo-sensitive device in the optocoupler OP_CO has one terminal coupled to a voltage Vr (e.g. 2V), and another terminal coupled to the primary reference ground. When load becomes lighter, the output voltage Vout goes up. The current Ip_sec, Ip_pri respectively flowing through the light emitting device and the photo-sensitive device both increases, and the voltage Vbst at the common node of the photo-sensitive device and the resistor R1 falls. If the voltage Vbst decreases to be lower than a threshold voltage Vth (e.g. 1.25V), the switching converter will enter into a burst mode to reduce switching loss.
Nevertheless, since the current Ip_sec and Ip_pri both become larger when the load goes lighter, more power loss is generated, which causes an adverse impact on light load efficiency and no load efficiency of the switching converter.